1. Field of the Invention
The present invention relates to a flat subwoofer of the type where typically a coil is arranged in an air gap, provided in a magnetic field, such that when the direction of the current through the coil changes direction, the magnetic field causes the coil to move back and forth in the air gap. By further connecting the coil to a membrane, the membrane will move back and forth, thereby generate airwaves which we perceive as sound.
Several loudspeakers are known in the art which are particularly designed to emit sounds having a relatively low frequency, the so-called bass sounds. The creation of this type of loud speakers normally requires a relatively large membrane in order to be able to provide a desired sound pressure which in turn requires that the membrane has a certain strength and stability in order not to wobble, float or deform during use. In order to address this it has traditionally been the custom to use membranes having a funnel or cone shape in that the movement back and forth of the membrane could be strengthened by the funnel shape of the membrane.
2. Description of the Related Art
For a number of applications it is desirable to minimize the extent of the construction along the axis along which the sound is emitted. In order to provide for the relatively flat constructions flat membrane parts have been used. The size of the membrane however creates a number of problems in that in order to provide a sufficient sound pressure the coil in the air gap is brought to move a relatively long distance which again, when a relatively flat and thereby unstable membrane is used, creates distortion, deformation and excessive load on the membrane, such that the design usually is a compromise between size of the unit and the quality of the sound emitted, in relation to the sound pressure which must be achieved.
From WO 2005/015950 is known a membrane construction where the membrane is kept relatively flat and stiff by means of folding the membrane material in a determined pattern. The folds will however distribute the emitted sound in a more diffuse manner, manner, such that the resulting sound impression, due to the distortion created by the membrane, will be a compromise between quality and constructional features of the loudspeaker.
Yet another example is known from US 2004188175 wherein a flat subwoofer membrane construction is disclosed, where the membrane is constructed of two connected parts. Each part is provided with a separate surround connecting the membrane part to the chassis in order to guide the membrane's movement which centrally is connected to a cylinder around which the voice coil is provided. In order to avoid wobbling of the voice coil in the air gap between the magnets and the pole pieces the two surrounds maintain the membrane in a very rigid position such that the amount of wobble of the cylinder and thereby the voice coil in the air gap is minimized. This construction, however, creates a number of problems, but mainly a relatively high effect loss is realised due to the fact that two separate surrounds are provided which need to be moved in order for the membrane to move whereby a relative, effective dampening of the membrane is achieved resulting in an overall power loss. Furthermore, wave propagation in the membrane will the cause the surrounds to act out of synchronization whereby wobbling of the cylinder and thereby the voice coil in the air gap will be the result which again will provide undesired sound emitting properties.